Applications requiring a beam fixed on a particular structure of the fundus to date have been hampered by the subject's normal eye movements. Uncontrolled eye movement introduces complications in such clinical treatments as laser photocoagulation. Thus a photocoagulation procedure could be improved using an accurate eye tracker to stabilize the photocoagulation beam at a particular site.
Successful prior art methods for tracking eye movements have evolved along two principal approaches. One early approach was to attach a tightly fitting contact lens to the surface of the eye, and to either attach a test object to the lens or to reflect an image from a front-surface mirror attached to the lens and through an optical system to produce an image of the fundus stabilized against eye movements. The degree of stabilization in opto-mechanical systems of this type is inherently limited by the slippage that occurs between the contact lens and the angular movements of the visual axis of the eye. Stabilization of this kind has been demonstrated to be insufficient for precise work.
A second procedure for image stabilization involves a two-component system, including a device for tracking the movements of the eye and a mechanism for moving the object or image proportionally. Historically, tracking eye movements in this manner involved following the movements of a contact lens attached to the eye, so that this method suffers the same limitation as the one first described.
The earlier efforts required accurately fitted individual lenses for each subject. Although the contact lens systems offer the best resolution of any system down to 10 arc sec, they do so in general at the sacrifice of range. They are normally applicable for the study of small eye movements. The expense and discomfort of the contact lens makes it a technique more suitable for use on a few subjects.
Another prior art class of instruments uses corneal reflections and reflections from other optical curvatures in the eye (Purkinje images), and measures translation, as well as rotation. However, these instruments track movements at the surface of the eye, and are less accurate for stabilizing an image at the back of the eye.
Recently, emphasis has been placed on tracking structures at the fundus. One fundus tracking method involves projecting a scanning pattern onto the eye fundus, and detecting the translational and rotational movements of the reflected pattern by means of high-speed correlation processing of the video signal. Scanning systems of this type have generally been "light starved". That is, the light intensity required to provide a good image signal-to-noise ratio exceeds acceptable retinal illumination levels. Furthermore, scanning systems require extremely regular and fast moving optical deflectors. As a whole these systems require complex electronic processing, limiting their response time.